Three-dimensional knowledge of a moving target during thoracic and abdominal radiotherapy is a key component to managing respiratory tumor motion, applying either motion inclusive, gated, or tracking treatments.
Several image guided radiotherapy (IGRT) systems were designed for the purpose of real-time 3D tumor position monitoring with synchronous stereoscopic imaging capability, such as four room-mounted kV source/detector pairs of the RTRT system and a dual gantry-mounted kV source/detector of the IRIS system.
Continuous real-time x-ray imaging is ideal for direct monitoring of the moving target. Significant imaging dose to the patient, however, is problematic. In an effort to reduce imaging dose to the patient during x-ray image-based tumor tracking, the 3D target position estimation employing an ‘internal-external’ correlation model was first introduced in synchrony of the CyberKnife system. Using a dual kV imager, the 3D target positions are determined occasionally by synchronous kV image pairs. With such measured 3D target positions and external respiratory signals, internal-external correlation is established which associates the external signal R(t) into each direction of internal target motion T(x, y, z; t), independently in a linear or curvilinear form. This well-established method is widely used clinically.
However, there are other kV imaging systems which do not allow synchronous stereoscopic imaging; the ExacTrac system has a dual kV imager sharing one generator alternately, and linear accelerators (linacs) for IGRT are equipped with a single kV imager rotating with the gantry. These systems are not currently used for respiratory tumor tracking.
What is needed is a general framework of correlation-based 3D target position estimation, which can be applied to synchronously acquired stereoscopic images and also sequentially acquired images, either by a dual kV imager alternately or by a single kV imager rotationally.